Integrated electrical circuits and other microstructured components are conventionally produced by applying a plurality of structured layers onto a suitable substrate which, for example, may be a silicon wafer. In order to structure the layers, they are first covered with a photoresist which is sensitive to light of a particular wavelength range, for example light in the deep ultraviolet (DUV, deep ultraviolet) spectral range. The wafer coated in this way is subsequently exposed in a microlithographic projection exposure apparatus. A pattern of structures, which is arranged on a mask, is thereby imaged onto the photoresist with the aid of a projection objective. Since the imaging scale used for this imaging is generally less than 1, such projection objectives are often also referred to as reducing objectives.
After the photoresist has been developed, the photoresist layer is washed off so that the wafer remains covered by the photoresist only at the exposed positions. The wafer is then subjected to an etching process or doped with foreign atoms, so that the layer lying below the photoresist is structured according to the pattern on the mask. The developed photoresist still remaining is subsequently removed from the wafer. This process is repeated until all the layers have been applied onto the wafer.
The performance of the projection exposure apparatus being used is determined not only by the imaging properties of the projection objective, but also by an illumination system which illuminates the mask with the so-called projection light. To this end, the illumination system contains a light source, for example a laser operated in pulsed mode, and a plurality of optical elements which generate light bundles, converging on the mask at field points within a usually elongate illumination field, from the light generated by the light source. The individual light bundles have particular properties, which in general are adapted to the projection objective.
These properties include inter alia the angular irradiance distribution of the light bundles which respectively converge at a point on the mask plane. The term angular irradiance distribution describes the way in which the overall intensity of a light bundle is distributed between the different directions in which the individual rays of the light bundle strike the relevant point in the mask plane. If the angular irradiance distribution is furthermore specially adapted to the pattern contained in the mask, then the latter can be imaged with higher imaging quality onto the wafer covered with photoresist.
Other important properties of the projection light are the intensity distribution within the illumination field and the contour of the illumination field per se. Since, in the so-called scanner mode of the microlithographic projection exposure apparatus, both the mask and the wafer to be exposed are moved continuously relative to the illumination field, the integrated total intensity with which a particular region is intended to be exposed can be established by the contour of the illumination field perpendicular to the direction of the motion. The contour may for example be varied via a finger aperture, the individual fingers of which can be driven individually and during active operation. The respective intensities along the illumination field may, for example, be recorded by an intensity sensor in the mask plane and converted into a sequence of measurement values. With a suitable arrangement, for example via a beam splitter, the integrated total intensities can thus be monitored during operation.
Recently, the use of so-called multi-mirror arrays (MMAs, also referred to as micromirror arrays or mirror matrices) has furthermore been envisaged for illumination systems of microlithographic projection exposure apparatus. These multi-mirror arrays include a multiplicity of individually drivable micromirrors, in order to deviate individual sub-beams of the projection light in different directions. With the aid of the micromirrors, the respective light sub-beams of the projection light can thus be directed individually onto different positions in a pupil surface of the illumination system. Since the intensity distribution in the pupil surface of the illumination system crucially influences the angular irradiance distribution of the projection light, the angular irradiance distribution can be set more flexibly owing to the individual drivability of the micromirrors. Particularly in connection with so-called unconventional illumination settings, in which an annular region or a plurality of poles are illuminated in the pupil surface, the use of multi-mirror arrays makes it possible to adapt the angular irradiance distribution to the respective circumstances, and in particular to the mask to be projected, without for example diffractive optical elements having to be replaced.
Multi-mirror arrays are often produced as microelectromechanical systems (MEMS) via lithographic methods, such as are known from semiconductor technology. The typical structure sizes are sometimes a few micrometres. Known examples of such systems are, for example, multi-mirror arrays whose micromirrors can be tilted digitally about an axis between two end positions. Such digital multi-mirror arrays are often used in digital projectors for showing images or films.
For use in the illumination system of a microlithographic projection exposure apparatus, the micromirrors should however be capable of quasi-continuously adopting every tilt angle within a working angle range. The actuators, which induce tilting of the micromirrors, may for example be configured as electrostatic or electromagnetic actuators. With known electrostatic actuators, for example, the tilting of the micromirrors is based on a stationary control electrode and a mirror electrode applied on the back side of the micromirror attracting one another with a different strength according to the applied voltage. Using a suitable suspension and a plurality of actuators, the micromirror can therefore be tilted through any desired tilt angle.
Owing to stringent desired properties for the accuracy when tilting the micromirrors, the actuators accordingly are driven extremely precisely by drive electronics. Here, it should be noted that owing to the large number of individual mirrors in a multi-mirror array, for example 1000, which usually are driven by a plurality of actuators per mirror, such drive electronics are designed very efficiently. Particularly when using a regulator system, the drive electronics are capable of processing a rapid sequence of measurement values in order to achieve a sufficient regulating frequency so that even high-frequency perturbations can be compensated for.
In analogy with the multi-mirror arrays described above, the use of so-called faceted mirrors having a multiplicity of individual mirror facets has been considered for modern X-ray lithography projection exposure apparatus in both the illumination system and the projection objective. Since it is desired to monitor the deformation of these faceted mirrors at a lot of different points, measurement values are generated in rapid succession and actuators of the faceted mirror have to be driven as a function of these measurement values.
All these devices and methods involve rapid evaluation and processing of a multiplicity of measurement values occurring as a rapid sequence of measurement values which for the most part are mutually independent.